Vibrio infection compromises byssal attachment in the mussel Mytilus coruscus by modulating energy homeostasis and byssal protein gene expression.

Mussels are important bivalve species that are cultured worldwide and attach to substrate by byssal threads. Mass detachment of mussels poses a significant threat to the sustainability of mussel aquaculture. It remains unclear whether biological factors such as infections caused by marine pathogenic bacteria affect byssal attachment in mussels. In this study, two strains each of Vibrio splendidus and V. anguillarum were isolated from seawater, and mussels were exposed to different concentrations (104, 105, 106, and 107 CFU/mL) of each bacterial strain for seven days. After exposure, we analyzed byssal thread number, detachment frequency, mechanical properties, adhesion strength, energy reserves (triglycerides, glycogen and proteins) in the gill and hepatopancreas, and the expression levels of byssal protein genes (mfp-1T, mfp-2, col-1, mfp-5 and mfp-6). The results showed that both Vibrio species caused a significant decrease in byssal thread number (P < 0.05), a significant increase in detachment frequency (P < 0.05), and a significant reduction in byssal diameter and breaking force (P < 0.05) compared with the control group after seven days of exposure. At 107 CFU/mL, both bacterial strains significantly decreased triglyceride levels in the hepatopancreas (P < 0.05), whereas protein and glycogen contents remained unchanged. qPCR analysis revealed that the byssal fiber structural gene col-1 was significantly downregulated at all bacterial concentrations (P < 0.05), and the adhesive plaque protein gene mfp-2 was significantly downregulated at 107 CFU/mL (P < 0.05). These findings demonstrate that Vibrio infection compromises byssal attachment by depleting hepatopancreatic triglyceride reserves and suppressing the expression of byssal protein genes col-1 and mfp-2. This study identifies Vibrio infection as a key biological factor underlying weakened byssal attachment in mussels and provides new insights into the ecological implications of host-pathogen interactions in bivalves.